Extraction of undesirable flavor and odor components from microbial cells

ABSTRACT

Microbial cells having a bland taste and improved odor characteristics for use in food preparations are prepared by solvent extraction. The process involves drying microbial cells after separation from a fermentation broth to a moisture content of 8 percent or less, extracting the dried cells with an aqueous alcoholic solvent containing 60 to 80 vol percent of an aliphatic alcohol having 1 to 3 carbon atoms per molecule, separating the solvent from the cells and drying the cells to yield a substantially solvent-free product. The dried product can be further treated by grinding, wetting with water and re-drying.

United States Patent Ridgway, Jr. et al.

1 EXTRACTION OF UNDESIRABLE FLAVOR AND ODOR COMPONENTS FROM MICROBIAL CELLS [75] Inventors: John A. Ridgway, Jr., La Porte.

lnd.; Kwei C. Chao, Naperville. Ill.

[73] Assignee: Standard Oil Company, Chicago, 111.

[22] Filed: May 3, 1974 [21] Appl. No.: 466,673

Related U.S. Application Data [63] Continuation-impart of Ser. No. 442,247, Feb. 13. 1974, which is a continuation-in-part of Ser. No. 285,848, Sept. 1, 1972, abandoned, which is a continuation-in-part of Ser. No. 64,866, July 24, 1970, abandoned.

[52] U.S. Cl. 426/60; 195/3 H; 195/82;

[51] Int. Cl..... A23j 1/18; Cl2c 11/28; Cl2c 11/30 [58] Field of Search 426/429, 364, 204, 148,

[56] References Cited UNITED STATES PATENTS 1.869721 8/1932 Sure 424/195 1 June 24, 1975 2,193,876 3/1940 Maizel 99/97 X 2,904,439 9/1959 Cooper ct 426/431 X 2.921.854 1/1960 Parker 426/147 3268412 8/1966 Champagnat ct a1 195/3 H 3.268419 8/1966 Champagnat ct a1 195/3 H Primary Examiner-David M. Naff Attorney, Agent, or FirmWerten F. W. Bellamy; Arthur G. Gilkes; William T. McClain [57] ABSTRACT Microbial cells having a bland taste and improved odor characteristics for use in food preparations are prepared by solvent extraction. The process involves drying microbial cells after separation from a fermentation broth to a moisture content of 8 percent or less, extracting the dried cells with an aqueous alcoholic solvent containing 60 to 80 vol percent of an aliphatic alcohol having 1 to 3 carbon atoms per molecule, separating the solvent from the cells and drying the cells to yield a substantially solvent-free product. The dried product can be further treated by grinding. wetting with water and re-drying.

23 Claims, No Drawings 1 EXTRACTION OF UNDESIRABLE FLAVOR AND ODOR COMPONENTS FROM MICROBIAL CELLS CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of our copending application Ser. No. 442.247. filed Feb. 13. 1974. which is a continuation-in-part of 285.848. filed Sept. 1. I972. now abandoned. which in turn is a continuation-in-part of 64.866, filed July 24. 1970. now abandoned.

BACKGROUND OF THE INVENTION In recent years much attention has been directed toward the development of new sources of protein which can be incorporated in foods or food additives suitable forhuman consumption. Rapid increases in world population have made the continued dependence on traditional sources of protein highly impractical. Moreover. the supply of protein from typical sources of protein. such as animal meat and certain vegetables. is inadequate to provide balanced diets sufficient to satisfy the tions of proteins and can be utilized directly in foods as a whole-cell material or can be treated to recover protein isolate. Recent efforts have shown that such microorganisms. grown on hydrocarbon substrates. can be successfully used in animal feeds. As yet these microorganisms have not been commercially accepted in food preparations suitable for human consumption.

. Single-cell protein (SCP) as normally produced has a distinctive flavor and odor that limits its use in foods intended for human consumption despite its admittedly attractive nutritional value. Part of the flavor problem appears to be associated with the lipids present in the microorganisms.

Past attempts to achieve acceptable flavor and odor characteristics in single-cell microbial products have employed both chemical and physical treatments. alone or sequentially. following an initial water wash of the harvested cells. For example. brewers yeast is debittered by washing with diluted alkaline reagents. In the processing of microorganisms grown on hydrocarbons. residual hydrocarbon substrate and lipid concentrations have been reduced by solvent extraction with organic solvents exhibiting good solubility for fats and sterols. As disclosed in US. Pat. No. 3.268.419 such solvents include alcohols. light hydrocarbons such as benzene. ethers, ketones. chlorinated solvents and liquefied petroleum gases. A preferred solvent system for removal of hydrocarbons as well as lipids comprises an azeotrope of hexane with ethanol or isopropanol. The water content of the microorganism cream is reduced by drying or by a prior washing with a polar solvent such as ethanol or isopropanol. Similarly. the process of US. Pat. No. 3.268.412 removes residual hydrocarbon substrate from ruptured microbial cell material or cell wall residue by extraction with a hydrocarbon or hydrocarbon-alcohol solvent optionally preceded by extraction with an alcohol such as ethanol.

Bland flavor characteristics have been achieved with vegetable protein. derived. for example. from de-oiled soybean material, by wetting with ethanol, dissolution in strong caustic alkali and finally precipitation with acid. Such extensive treatment. as disclosed in US. Pat. No. 3,043,826,. adds significantly to the cost of product. especially because only a minor portion of the total vegetable protein is recovered in the purified form.

In an extraction process affording a food-grade protein-rich product care must usually be taken to remove all of the organic extraction solvent. Accordingly, the choice of solvent is limited to those possessing a suitably high volatility preferably coupled with a very low toxicity. One exceptional situation exists where ethanol. an accepted food-grade substance in itself, is employed as the extractant.

Such treatments tend to involve a plurality of processing steps which add significantly to the cost of the food or food ingredient thereby produced. A need has continued to exist for an inexpensive, simple yet effective process for affording an acceptable quality foodgrade SCP material for human consumption.

SUMMARY OF THE INVENTION One object of our invention is to provide an improved process for removal of undesirable flavor and odor components from single-cell, protein-containing microbial products.

Another objectof our invention is to provide a bland single-cell protein (SCP) material of a quality suitable for use in foods; intended for human consumption.

The process of our invention is broadly applicable to SCP materials and is especially applicable to bacteria and yeasts with particular emphasis on the latter.

In one aspect of the practice of this invention, the single-cell protein materials are heat-killed prior to its use in the present process.

The single-cell protein materials are heat-killed by subjecting the said materials to a pasteurization process prior to dryingthe material to a moisture content no greater than about 8 wt. percent.

In orderto heat-kill the viable cells of the single-cell protein material by pasteurization, the said material is heated l for. thiry (30 seconds at temperatures ranging from about 159F to 178F; (2) for one (1) minute at temperatures ranging from about 156F to 174F; or (3) for five (5) minutes at from about 15 lF to 168F.

In another aspect of the present invention, the singlecell protein mateials derived from single-cell microbial organisms grown by fermentation on a suitable substrate are not heat-killed prior to using the same in this process. The viable cells of the single-cell protein material are killed during the extraction step of the present process. When the rviable cells come in contact with the aqueous aliphatic alcohol extractant which contains from 60 to vol. percent alcohol. the said cells are killed almost immediately.

Whether the viable cells of the single-cell protein material are heat-leilled by pasteurization or killed almost immediately when contacted with the aqueous aliphatic alcohol extractant containing 60 to 80 vol. percent alcohol. dead cells are produced in both instances.

We have found that undesirable flavor components can be extracted from single-cell microbial material by use of an extraction solvent comprising aqueous alcohol. where the alcohol contains 1 .to 3. carbon atoms per molecule and the alcohol solvent contains 20 to 401 01. percent water. The SCP material is not degraded as to ,other properties and, as recovered in good yield. has a bland flavor. Odor properties are similarly improved.

Our invention is particularly applicable to the afford- .ing. of a bland-tasting SCP material when operating upon previously dried single-cell microbial organisms grown 0.1 an ethanol-containing substrate with an alcoholic solvent comprising methanol, ethanol. n-propanol or isopropanol. v y I i Other yeasts and bacteria grown upon a variety of substrates have been improved substantially in respect of taste and odor qualities when treated by the process of this invention. i

DEscRIRTIoN OF PREFERRED EMBODIMENTS I t ha;ve -been minimized. Improved digestibility has been indicated byfeeding tests, increasedshelf life has been observed, and increased holding capacity for water and oilisobserved when treated SCP. material is incorporated inlground beef. g 1

Any microbial cell material can be treated according to the extraction process of this inventionln a fully integrated. continuous. system, the microbial cells are conveniently grown in a fermentation stage where oxygen and a suitable substrate, such as a liquid or gaseous hydrocarbon, an alcohol or a carbohydrate together with a nutrient solution containing vitamins and miner- .als are fed to a stirred reactor containing the microorganisms. The growth rate of microorganisms on the substrate is typically exponential in nature. As the mig croorganism concentration increases, a portion of the fermentation broth is withd'rawn from the stirred reactor and the cellular microorganisms separated therefrom. Although a pasteurized orunpasteurized cell paste concentrate may be employed in the process of this invention, best results have been obtained when treating a" pasteurized.or unpasteurized product that has. preyiously been isolated and dried; i.e., brought to a moisture content no greater than about 8 wt. percent andpreferably about 5 wt. percent.

By way of illustration, bacteria such as those listed in Tablel and yeasts suchas, those listed in Table II are suitable microorganisms for use as-starting materials in the practice'of this invention. 7

TAEiLE l SUITABLE BACTERIA' Candida tgurwl/u -Continued The extraction process of, this invention may employ any feasible arrangement ofgequipment but should generally provide for a contacting zone, a separation zone i and a solvent recovery zone. ,The contacting zone may comprise a simple mixing vat, an agitated vessel, a baffled rotating drum, a screw-feed countercurrent flow contac tor, a percolation vessel, or other appropriate solid-liquid contacting device. In a preferred arrangement glass-lined vessels and stainless steel piping and fittings are 'employed.

Thecontacting step may be effected in stages and may be operated in either a. batch or continuous manner. Separation of purified SCP material may be effected by filtration, decantation, centrifugation or other means. Combination filtration-extraction equipment may also be employedf lf staging is not employed there should preferably be provision made for washing the extracted SCP material with fresh solvent to maximize separation of the extract components.

ln the typical operation of this invention at least a portion of the solvent will be recovered for reuse. This may be done by vaporization and condensation, as in a "flash distillation apparatus, whereby the alcoholwater solvent is taken overhead and an extract material is recovered as a bottoms product.

The SCP material, purified by extraction with aqueous alcohol, must be freed of' residual alcohol, except possibly w hen ethanol hasbeen'ernployed in the extraction solvent. This drying step'may be effected in any typical desolventizing equipment. In a preferred operation the wet cell product is worked mechanically during the desolventizing to avoid theformation of large agglomerates. Coarse cell product particles should pass through an 8-mesh'standard screen. Both oven-drying and spray-drying techniques maybe employed with proper selection of time and temperature (generally in the range from to 100C.) dependent upon the de- 2 si red final moisture content and physical form of the purified SCP material. ln some instances the dried material may require a further moistening with water and a redrying operation, preferably in a drying oven at about80 to "l0()C If desired. a fine-grinding step may be included to affordparticles passing through screens in the range from 50-.to l00-meshfi Where feasible, provision may' bc made ,for recovery vaporized solvent for combination with other recove red solvent for recycle to the contacting zone. Where stripping of purified SCP material has been effected .with an ioxygemcontaining gas stream and the alcohol component of the solvent is also employedin the fermentation substrate, the effluent stripping gas may be directed to the fermentor to provide both oxygen and alcohol for microbial cell growth.

The alcohols suitable for use in our invention are the lower aliphatic alcohols which are completely miscible with water and are sufficiently volatile to permit removal simply from purified, extracted microbial cell material without exposure to severe processing conditions. We have found that suitable alcohols include methanol, ethanol and the propanols. A preferred alcohol is ethanol.

The extraction process employing an alcoholic solvent is most effective in producing a bland tasting SCP material when about to 25 wt. percent of the original microbial cell material (dry basis) is removed. The removed material comprises fats, sterols, phospholipids, nonprotein nitrogenous compounds and others that contribute to the customary undesirable taste and odor qualities of yeasts and bacterial products. Anhydrous alcohol has been found to remove only a portion of these undesirable components while dilute aqueous alcohol not only is less selective for the undesirable components but also leaves the extracted solids in a flocculent state not readily filterable. We have found that the optimum extraction of undesirable components is achieved when employing a C -C alcohol, preferably ethanol, together with to 40 vol. percent water. In a quite unexpected manner, this particular range of a1- cohol and water concentrations also affords the best separation characteristics in the slurry of cells and solvent. Accordingly, the solvent system for the extraction process of our invention should be formulated to contain 60 to 80 vol. percent of the appropriate alcohol, the remaining portion being water.

The process of our invention may be operated at any temperature within the range from room temperature (about 20C) up to about 100C., say within the range from 20 to 80C., preferably within the range from 50 to 80C. Generally, the preferred temperature will be no higher than the atmospheric boiling point of the selected alcohol-water azeotrope although, where the extraction equipment permits, super-atmospheric pressure may be employed in order to achieve a higher extraction temperature.

The time required for effecting the desired depth of extraction will vary inversely with the degree of dispersion of the solid in the solvent but will usually be at least 0.1 hour, but within the orange from 0.5 to 5.0, preferably 0.1 to 2.0 hours. Where staging is employed the time employed in successive stages may be varied but the total time will usually fall within the above limits.

The weight ratio of solvent to solid effective for achieving the desired depth of extraction will vary inversely with the temperature employed but will usually be within the range from 3:1 to 7:1.

The process of this invention may be employed to improve the food quality of any appropriate single-cell microorganism material. In particular, our process has been employed successfully in improving the flavor and odor quality characteristics of various yeasts grown on a variety of substrates. These yeasts include brewers yeasts (Sacclmrumyces celevisiae), Torula yeast grown on waste sulflte liquor, a yeast of the genus Candida grown on a paraffinic gas-oil hydrocarbon substrate, and a yeast (Candida utilis) which has been treated under conditions whereby its nucleic acid content had been reduced. In each instance, aqueous alcohol ex traction according to the process of this invention gave a product having a bland taste and, overall, improved flavor and odor properties.

Surprisingly, similar improvement was found with respect to extract yield, ease of extraction and separation, and improvement in flavor and odor qualities when the yeast cells contained in a paste or aqueous slurry had first been broken, as by use of a French press. It appears that the water-dispersible cell protein is coagulated in the presence of the aqueous alcohol and thereafter behaves in this process an any other whole cell material and is not lost as extract. Treatment of the alcohol-extract ruptured-cell material with sodium hydroxide yields a sodium proteinate having improved flavor.

Our process may be applied to single-cell microorganisms immediately after harvesting, after washing, after dewatering or drying, or after other processing operations but preferably after drying. When applied to a pasteurized or unpasteurized cell-cream or cell paste the volume of water associated therewith must be known so that sufficient alcohol or alcohol-water azeotrope may be added to provide the desired alcohol concentration in the initial solvent extraction stage.

A preferred application of our invention relates to the improved quality properties developed in the yeast cells of the strain Candida utilis as grown on an ethanol substrate and dried prior to extraction treatment. In this applicationethanol-water, preferably about vol. percent ethanol 30 vol. percent water, is employed as solvent so that a common, non-toxic alcohol is present both as substrate and as extractant. This system affords an improved flavor and odor in the extracted yeast product, introduces no extraneous substance having a toxic nature, and simplifies the recovery of solvent. A portion of the extract solution may be fed to the fermentor without further treatment, other than possibly a sterilization step, to provide the required substrate carbon source. The remainder of the extract solution may be sent to solvent recovery in the usual fashion. when drying the extracted cell material product any ethanol vapors may be recovered from a drying air stream or, optionally, the ethanol vapor-air mixture may be charged to the fermentor to supply both the requisite oxygen and additional substrate material.

EXAMPLES The following examples are illustrative of our invention.

EXAMPLE I Drum-dried Torula yeast (Candida utilis) having a distinctive yeasty odor and taste, was subjected to extraction with each of the solvents shown in Table 111 for a period of about 16 hours. These extractions were conducted at the boiling points of the respective solvents in a Soxhlet extractor (except as noted). Among these solvents only ethanol gave a definite improvement in flavor but the desired bland flavor character was not achieved.

EXAMPLE ll Spray-dried Torula yeast (Candida utilis) was subjected to a series of qualitative extraction experiments employing ethanol, water and mixtures thereof. The yeast was mixed with the test solvent at 50C., allowed to settle and finally filtered through filter paper. The examination of the extracts and filtrates is summarized in Table IV. Good settling and rapid filtration. coupled with extraction of a relatively large amount of material from the yeast. was observed only when the extraction solvent contained 60 or 80 vol. percent ethanol.

EXAMPLE III Extraction studies were conducted with ethanolwater mixtures containing 60, 70 and 80 vol. percent ethanol. Yeasts examined included both laboratory and commercial samples of spray-dried Torula yeasts, as well as yeast paste either taken directly from the centrifuge or subjected thereafter to cell rupture by extrusion from a French press. Extractions were conducted in a Soxhlet extractor for about 16 hours. Composition of the extractor charge was calculated to provide the desired alcohol concentration in the extraction zone. After extraction was terminated, the yeast residue was dried at 100C. The extract material was recovered from the solvent by evaporation on a steam bath. Results of these studies are summarized in Table V.

Marked improvement in flavor was noted in all instances, including that of the comparatively poor quality commercial Torula yeast. Extract yield and ease of extraction were not affected by first breaking the cells.

EXAMPLE IV Spray-dried Torula yeast (Candida urilis) was subjected to a series of extraction experiments employing isopropanol alone and in mixtures with water. In each experiment 1 gram of yeast was contacted with 50 ml. solvent at room temperature, allowed to settle and finally filtered through filter paper. The examination of the cake, filtrate and extract is summarized in Table VI. Based on filtration performance and rate of re-settling, coupled with extraction of a substantial amount of material from the yeast, the optimum solvent composition contained 60 to 80 vol. percent isopropanol.

EXAMPLE V The extraction technique described in Example 111 was employed with debittered brewers yeast, Torula yeast grown on waste sulfite liquor, Torula yeast grown on a gas-oil hydrocarbon fraction, and a yeast (Candida milis) having a. reduced nucleic acid content. In each instance marked improvement in flavor of the extracted yeast was observed. Additionally, the process of this Example removed the bitter hop flavor from fresh brewers yeast, thus avoiding the necessity for an alkaliwashing step.

EXAMPLE VI Cells of the bacterial strain Nac'ardia sp., grown on a butane substrate, were extracted at 4550C. for 0.5 hour employing as extraction solvent aqueous isopropanol containing 20 vol. percent water. Marked improvement was noted in color and odor of the extracted cells.

EXAMPLE VII The technique of Example VI was employed with isopropanol containing 40 vol. percent water. A similar improvement was noted in the color and odor of the extracted cells.

EXAMPLE VIlI Spray-dried Torula yeast (230 g.) was treated with two 1200 ml. portions of a vol. percent isopropanol- 30 vol. percent water mixture at 65C. The extracted product was then dehydrated by contacting with a 1200 ml. portion of anhydrous isopropanol. After drying for 12 hours at 50C. and finally drying at 100C. the yeast possessed a flavor objectionable only due to the presence of a residual solvent taste. Subsequent wetting with water and re-drying at 100C. removed the objectionable solvent taste.

EXAMPLE IX A slurry containing 19 wt. pecent yeast cells (Candida utilis) in aqueous ethanol vol. percent ethanol) was charged continuously to the top of a countercurrent extraction column at the rate of 370 g./hour cells. The same aqueous ethanol solvent composition was charged upflow in the extractor at the rate of 3000 ml./hour. Column temperature was maintained at about 65C. Extracted cells were recovered after filtration at the rate of 296 g./hour as a cake containing about 50 wt. percent cells in solvent. Effluent solvent from the top of the extractor was purified by flashing and condensed for recycle. Extract was recovered from the flash bottoms at the rate of 74 g./hour together with some solvent.

After removal of alcohol, the extract separated into a waxy phase and a water-soluble phase. The waxy phase contained phospholipids.

The recovered cell cake was broken up and dried for 12 hours at 50C. After coarse grinding to pass a 9- mesh screen, the extracted cell material was dried further for 12 hours at 50C. and finally fine-ground to pass through 0.01 inch slots.

In an alternate procedure the coarse-ground material was wetted with water, dried at 50C., ground to pass through 0.01 inch slots and finally dried in an oven at C. This product exhibited a preferred combination of properties for use in foodstuffs intended for human consumption, including bland taste, minimum odor, high protein efficiency ratio in feeding tests, good storage stability and excellent holding capacity for water or oil in meat formulations.

TABLE IV Extraction of Spray-Dried Torula Yeast with Ethanol-Water Solvent Character of Mixture Character of Filtration Character vol. 7: Ethanol Supernatant Phase Cake Volume" Filtrate Filtration time. minutes" of Extract 100 Cloudy l Clear. Small. oily water-white 80 Clear. 5 Clear. light 6 Substantial. solid slightly yellow yellow- 60 Clear. yellow 3 Clear. yellow 6 Large. solid 40 Cloudy. 2 Clear 60 Medium. brittle slightly yellow solid Cloudy. 14 Cloudy 60 slightly yellow 0 Cloudy. 3 Cloudy 60 slightly yellow "An arbitrary rating: 1 small volume. 14 large volume with little settling TABLE V Extraction of Torula Yeast with Ethanol-Water Solvent Extract vol. 7: ethanol Yield. Wt. 7( Description Character of Residue Spray-Dried Yeast 60 20.0 Brown. gummy solid Hard solid. bland taste 70 17.5. 18.0. 18.6 Friable solid. bland taste" 70 19.2 Black. gummy solid Friable solid. improved flavor" 70" 19.5 Brown. gummy solid Friable solid. improved flavor but noticeable ammonia taste 80 17.4 Friable solid. bland taste 20.6 Tan solid" Yeast Paste from Centrifuge Similar to spray-dried series 70 18.0 Yeast Paste from French Press Similar to spray-dried series "Nitrogen analysis indicated a higher nominal protein content than in the charge material. Commcrcial Torula ycast employed in this experiment.

Solvent also contained 371 ammonia. "Dried under vacuum at room temperature.

TABLE VI Extraction of Torula Yeast with lsopropanol-Water Solvent. Settling Rate" Extract Residue vol. 7: Super- Concentration Character Filter Settled Relative isopropanol natant in of rate" cake re-settling color supernatant solid depth" rate" g/ ml.

16 yellow 0.1 1 finely divided 120 24 3 22 0.10 77 22 2" 25 0.10 57 22 l 9 0.08 heavy curd 54 28 l 5 0.07 34 33 l 3 0.03 39 44 2 46 clear 0 finely divided 142 9 3 "Mm. supernatant liquid developing in test tube in IS minutes. Time in seconds for 10 ml. to pass through thc filter. "Measured in thirty-seconds of an inch. "Arbitrary scale: 1 good. 3 poor Supernatant liquid as cloudy.

We claim: 1. A process for improving the flavor characteristics of single-cell microbial organisms. comprising the steps of:

60 a. drying said single-cell microbial organisms to a moisture content no greater than about 8 wt. percent;

b. preparing a slurry of the dried single-cell microbial organisms with an aqueous alcoholic solution con- 65 taining 60 to 80 vol. percent of an aliphatic alcohol having 1 to 3 carbon atoms per molecule in an amount to provide a weight ratio of alcoholic solution to single-cell microbial organisms within the range from 3:1 to 7:1, whereby the viable cells are killed when mixed with said alcoholic solution;

1 l c. agitating the slurry at a temperature in the range from 20 to 100C. for a period of 0.1 to 2.0 hours; (1. filtering the slurry to recover extracted single-cell microbial organisms; and e. drying the extracted single-cell microbial organisms to yield a substantially solvent-free product. 2. The process of claim 1 wherein the aliphatic alcohol is ethanol.

3. The process of claim 1 wherein the single-cell microbial organism is a yeast product.

4. The process of claim 3 wherein the yeast is selected from the group consisting of Sac'cliaramyces verevisiae, Saccharomyces carlsbergensis. Saccllaramyces fragilis and Candida utilis.

5. The process of claim 4 wherein the yeast is Candida utilis.

6. The process of claim 1 wherein the single-cell mi crobial food product is a bacteria product.

7. The process of claim 1 wherein the substantially solvent-free product is additionally ground to a fine particle size, wetted with water and dried at a temperature in the range from 50 to 100C.

8. The process of claim 1 wherein the slurry is agitated at a temperature in the range from 50 to 80C.

9. The process of claim 1 wherein the extracted material from step (d) is l slurried with an aqueous alcoholic solution containing 60 to 80 vol. percent of an aliphatic alcohol having 1 to 3 carbon atoms per molecule in an amount to provide a weight ratio of alcoholic solution to a single-cell microbial organisms within the range from 3:1 to 7:1; (2) agitated at a temperature in the range from 20 to 100C. for a period of 0.1 to 2.0 hours; and (3) filtered to recover an extracted singlecell microbial organisms.

10. A process for upgrading single-cell protein materials derived from single-cell microbial organisms grown by fermentation on a suitable organic substrate and intended for use in food products, comprising the steps of:

a. isolating a harvest of said cells from the fermentation broth;

b. washing the harvested cells with water;

c. dewatering the washed cells to yield a cell paste;

d. drying the cell paste to a moisture content no greater than about 8 wt. percent.;

e. slurrying the dried cells with an aliphatic alcohol, selected from the group consisting of methanol, ethanol, n-propanol and isopropanol, and water to provide an aqueous alcoholic solution containing 60 to 80 vol. percent alcohol and in an amount selected to provide a weight ratio of alcoholic solution to dried cells within the range from 3:1 to 7:1, whereby the viable cells are killed when mixed with said alcoholic solution;

f. extracting the cells by contacting with the aqueous alcoholic solution at a temperature in the range from 20 to 100C. for at least 0.1 hour;

g. separating the extracted cells from the aqueous alcoholic solution; and

h. drying the extracted cells to yield a substantially solvent-free product.

11. The process of claim 10 wherein the aliphatic alcohol is ethanol.

12. The process of claim 10 wherein the harvested cells of single-cell microbial organisms comprise a yeast selected from the group consisting of Sacclzaramyces cerevisiae. Saccharomyces c'arlsbergensis, Sacclzaromyces fragilis and Candida utilis.

13. The process of claim 12 wherein the yeast is Candida ulilis. 7

14. The process of claim 10 wherein the harvested cells of single-cell microbial organisms comprise bacteria. I

15. The process of claim 10 wherein the cells are contacted with the aqueous alcoholic solution at a temperature in the range from 50 to C. for a period of 0.1 to 2.0 hours.

16. A process for the production of purified singlecell protein material from single-cell microbial organisms, grown in a fermentor on a substrate comprising aqueous ethanol and afforded as a water-washed and dried cell paste, comprising the steps of:

a. drying said single-cell microbial organisms to a moisture content no greater than 8 wt. percent;

b. diluting the said dried cell paste with sufficient ethanol-water azeotrope and additional water to provide a slurry of microbial cells in aqueous ethanol containing 60 to 80 vol. percent ethanol, said slurry providing a weight ratio of aqueous ethanol to microbial cells within the range from 3:1 to 7: 1 whereby the viable cells are killed when mixed with said alcoholic solution;

c. agitating the slurry for a period of 0.5 to 5.0 hours at a temperature in the range from 20 to 80C d. filtering the slurry to provide an aqueous ethanolic extract solution and a purified single-cell protein material;

e. separating the aqueous ethanolic extract solution into two portions;

f. distilling the first portion of extract solution to recover overhead an ethanol-water azeotrope;

g. cycling the recovered ethanol-water azeotrope to the dried cell paste dilution step; and

h. drying the purified single-cell protein material to yield a substantially solvent-free product.

17. A process of claim 16 wherein the remaining portion of extract solution from step (d) is fed to the fermentor to provide substrate for single-cell microbial growth.

18. The process of claim 16 wherein the purified single-cell protein material is dried in a drying zone in a stream of an oxygen-containing gas at a temperature in the range from 50 to C. and the effluent gas stream from the drying zone containing additional recovered ethanol, is fed to the fermentor.

19. The process of claim 16 wherein the dried purified single-cell protein material is subjected to further treatment comprising the steps of:

a. grinding the dried cell material to afford coarse particles capable of passing through an 8-mesh screen;

b. adding sufficient water to thoroughly moisten the coarse-ground particles;

0. re-drying the moistened particles at a temperature of about 50C;

d. re-grinding the re-dried particles to afford fine particles capable of passing through a SO-mesh screen;- and e. further drying the fine-ground particles at a temperature in the range from about 50 to 100C.

20. The process of claim 16 wherein the single-cell microbial organisms are yeast cells.

21. The process of claim 20 wherein the yeast is selected from the group consisting of Saccharomyces cerevisiae. Sacc/zaromyces c'arlsbergensis, Saccharomyces fragill's and Candida uIiIis.

22. The process of claim 21 wherein the yeast is Candia'a utilis.

23. The process of claim 16 wherein the single-cell microbial organisms are bacteria.

UNITED STATES PATENT ()VFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3,891,772

DATED 1 June 24, 1975 |NVENTOR(S) John A. Ridgway, Jr., and Kwei C. Chao I It is certified that error appears in the above-identified patent and that said Letters Patent are'hereby corrected as shown below:

Column 2, line 50, "mateials" should be materials 3 26, "with" should be While 5 48, "oramge' should be range 63, "celevisiae" should be cerevisiae 6 l4, "alcohol-extract" should be alcohol-extracted 8 20, pec zent" should be percent 9) Footnote b should be added at bottom of Table IV.

" ll 30, "a single-cell" should be single-cell o- Signed and Scaled this twenty-first D ay Of October 1975 [SEAL] Attest: O

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner uflaren'ts and Trademarks 

1. A PROCESS FOR IMPROVING THE FLAVOR CHARACTERISTICS OF SINGLE-CELL MICROBIAL ORGANISM, COMPRISING THE STEPS OF: A. DRYING SAID SINGLE-CELL MICROBIAL ORGANISMS TO A MOSTURE CONVENT NO GREATER THAN ABOUT 8 WT. PERCENT, B. PREPARING A SLURRY OF THE DRIED SINGLE-CELL MICROBIAL ORGANISMS WITH AN AQUEOUS ALCOHOLIC SOLUTION CONTAINING 60 TO 80 VOL. PERCENT OF AN ALIPHATIC ALCOHOL HAVING 1 TO 3 CARBON ATOMS PER MOLECULE IN AN AMOUNT TO PROVIDE A WEIGHT RATIO OF ALCOHOLIC SOLUTION TO SINGLE-CELL MICROBIAL ORG ORGANISMS WITHIN THE RANGE FROM 3:1 TO 7:1, WHEREBY THE VIABLE CELLS ARE KILLED WHEN MIXED WITH SAID ALCOHOLIC SOLUTION, C. AGAINTGING THE SLURRY AT A TEMPERATURE IN THE RANGE FROM 20* TO 100*C. FOR A PERIOD OF 0.1 TO 2.0 HOURS: D. FILTERING THE SLURRY TO RECOVER EXTRACTED SINGLE-CELL MICRO BIAL ORGANISMS, AND E. DRYING THE EXTRACTED SINGLE-CELL MICROBIAL ORGANISMS TO YIELD A SUBSTANTIALLY SOLVENT-FREE PRODUCT.
 2. The process of claim 1 wherein the aliphatic alcohol is ethanol.
 3. The process of claim 1 wherein the single-cell microbial organism is a yeast product.
 4. The process of claim 3 wherein the yeast is selected from the group consisting of Saccharomyces cerevisiae, Saccharomyces carlsbergensis, Saccharomyces fragilis and Candida utilis.
 5. The process of claim 4 wherein the yeast is Candida utilis.
 6. The process of claim 1 wherein the single-cell microbial food product is a bacteria product.
 7. The process of claim 1 wherein the substantially solvent-free product is additionally ground to a fine particle size, wetted with water and dried at a temperature in the range from 50* to 100*C.
 8. The process of claim 1 wherein the slurry is agitated at a temperature in the range from 50* to 80*C.
 9. The process of claim 1 wherein the extracted material from step (d) is (1) slurried with an aqueous alcoholic solution containing 60 to 80 vol. percent of an aliphatic alcohol having 1 to 3 carbon atoms per molecule in an amount to provide a weight ratio of alcoholic solution to a single-cell microbial organisms within the range from 3:1 to 7:1; (2) agitated at a temperature in the range from 20* to 100*C. for a period of 0.1 to 2.0 hours; and (3) filtered to recover an extracted single-cell microbial organisms.
 10. A process for upgrading single-cell protein materials derived from single-cell microbial organisms grown by fermentation on a suitable organic substrate and intended for use in food products, comprising the steps of: a. isolating a harvest of said cells from the fermentation broth; b. washing the harvested cells with water; c. dewatering the washed cells to yield a cell paste; d. drying the cell paste to a moisture content no greater than about 8 wt. percent.; e. slurrying the dried cells with an aliphatic alcohol, selected from the group consisting of methanol, ethanol, n-propanol and isopropanol, and water to provide an aqueous alcoholic solution containing 60 to 80 vol. percent alcohol and in an amount selected to provide a weight ratio of alcoholic solution to dried cells within the range from 3:1 to 7:1, whereby the viable cells are killed when mixed with said alcoholic solution; f. extracting the cells by contacting with the aqueous alcoholic solution at a temperature in the range from 20* to 100*C. for at least 0.1 hour; g. separating the extracted cells from the aqueous alcoholic solution; and h. drying the extracted cells to yield a substantially solvent-free product.
 11. The process of claim 10 wherein the aliphatic alcohol is ethanol.
 12. The process of claim 10 wherein the harvested cells of single-cell microbial organisms comprise a yeast selected from the group consisting of Saccharomyces cerevisiae. Saccharomyces carlsbergensis, Saccharomyces fragilis and Candida utilis.
 13. The process of claim 12 wherein the yeast is Candida utilis.
 14. The process of claim 10 wherein the harvested cells of single-cell microbial organisms comprise bacteria.
 15. The process of claim 10 wherein the cells are contacted with the aqueous alcoholic solution at a temperature in the range from 50* to 80*C. for a period of 0.1 to 2.0 hours.
 16. A process for the production of purified single-cell protein material from single-cell microbial organisms, grown in a fermentor on a substrate comprising aqueous ethanol and afforded as a water-washed and dried cell paste, comprising the steps of: a. drying said single-cell microbial organisms to a moisture content no greater than 8 wt. percent; b. diluting the said dried cell paste with sufficient ethanol-water azeotrope and additional water to provide a slurry of microbial cells in aqueous ethanol containing 60 to 80 vol. percent ethanol, said slurry providing a weight ratio of aqueous ethanol to microbial cells within the range from 3:1 to 7:1, whereby the viable cells are killed when mixed with said alcoholic solution; c. agitating the slurry for a period of 0.5 to 5.0 hours at a temperature in the range from 20* to 80*C; d. filtering the slurry to provide an aqueous ethanolic extract solution and a purified single-cell protein material; e. separating the aqueous ethanolic extract solution into two portions; f. distilling the first portion of extract solution to recover overhead an ethanol-water azeotrope; g. cycling the recovered ethanol-water azeotrope to the dried cell paste dilution step; and h. drying the purified single-cell protein material to yield a substantially solvent-free product.
 17. A process of claim 16 wherein the remaining portion of extract solution from step (d) is fed to the fermentor to provide substrate for single-cell microbial growth.
 18. The process of claim 16 wherein the purified single-cell protein material is dried in a drying zone in a stream of an oxygen-containing gas at a temperature in the range from 50* to 100*C. and the effluent gas stream from the drying zone containing additional recovered ethanol, is fed to the fermentor.
 19. The process of claim 16 wherein the dried purified single-cell protein material is subjected to further treatment comprising the steps of: a. grinding the dried cell material to afford coarse particles capable of passing through an 8-mesh screen; b. adding sufficient water to thoroughly moisten the coarse-ground particles; c. re-drying the moistened particles at a temperature of about 50*C; d. re-grinding the re-dried particles to afford fine particles capable of passing through a 50-mesh screen; and e. further drying the fine-ground particles at a temperature in the range from about 50* to 100*C.
 20. The process of claim 16 wherein the single-cell microbial organisms are yeast cells.
 21. The process of claim 20 wherein the yeast is selected from the group consisting of Saccharomyces cerevisiae, Saccharomyces carlsbergensis, Saccharomyces fragilis and Candida utilis.
 22. The process of claim 21 wherein the yeast is Candida utilis.
 23. The process of claim 16 wherein the single-cell microbial organisms are bacteria. 